This invention relates to the field of direct reduction of iron oxide materials to produce metallic iron in a solid state, often refered to as direct reduced iron, or DRI. Many processes have, in recent years, utilized solid carbonaceous materials, fuel oils, natural gases and coal gases as a source of reductant for direct reduction. Batch processes, continuous processes and semicontinuous processes with cyclio process conditions, have all been considered to be state of the art commercialized processes, treating iron oxide in the form of pellets, natural lump ores, and fines, through means of shaft furnaces, kilns, rotary kilns, fluidized bed furnaces, and batch kilns.
Although great amounts of time, effort and money have been spent on these developments and some installations have been commercially operated, all the units have failed to meet commercially acceptable operation without difficulty, have had inherent design difficulties, have not had sufficient flexibility to use varying qualities of fuel, have not been accepted in industrial oriented societies, and have undergone major technological change due to inefficiencies and design. These processes also have not had the flexibility of utilizing low cost iron oxide raw material sources available from local sources in any form, therefore have not been commercially acceptable in some areas.
In recent years, as the quality requirements and specifications on finished iron and steel products have become of paramount importance, the need by the iron and steel industry for an economic source of high quality melting iron, which is sufficiently flexible to utilize various sources of and qualities of both iron oxide and fuels has become increasingly important for the survival of the industry. Tramp elements are present in most recycled materials and the availability of high quality melting stock is diminishing. Thus, a highly efficient and flexible direct reduction process is highly desirable. Direct reduced iron in the form of pellets, lump ore or agglomerated fines are generally free of undesired tramp elements as during the process of reduction the oxygen in iron oxide is removed without affecting the oxygen-bearing tramp elements in its solid state and is ideal for melting in electric arc vessels, oxygen blowing vessels, blast furnaces, and foundries for the iron and steel industry. The inhibiting factor to utilizing such direct reduced iron on a commercial scale has been principally the economics of producing direct reduced iron with a high degree of efficiency, inherent design deficiencies in commercially available processes, and the lack of flexibility in use of available iron and fuel sources.
The present invention is a direct reduction process in which reducing gas, controlled in quality content of reducing agent and hydrogen to carbon monoxide ratios, is produced by a continuous catalytic reforming gaseous hydrocarbon, gaseous in the natural state or vaporizable gases, to produce a controlled ratio of hydrogen and carbon monoxide and in which the oxidant utilized for hydrocarbon reforming is carbon dioxide and water vapor formed in the reduction process and steam generated from waste process and/or waste flue gases, resulting in optimum thermal efficiency of the process, optimum utilization of equipment, flexibility in use of varying sources of fuel and flexibility of raw material sources, and capable of utilizing a variety of types of reduction furnace units.
At the present time, a direct reduction process used commercially in some areas of the world where hydrocarbon fuels are available at low cost requires a ratio control of hydrogen and carbon monoxide at the discharge of the catalytic reformer and prior to injection into the reduction furnace. The present method of controlling the ratio is to saturate the process recycled gas with recycled hot water through a gas scrubber, resulting in contact of contaminated water consisting of acids, sulphur compounds and solids to downstream equipment, having a deleterious effect on that equipment. Additionally, the saturation of the recycled process gases at the required ratio of hydrogen and carbon monoxide decrease the efficiency of the process gas handling equipment.
As waste flue gases are ejected from the catalytic reformer in excess energy, the concept of the present invention is to recover the waste heat having the following effects on or capabilities of the process:
1. Generation of steam for hydrogen and carbon monoxide ratio control of the reducing gas.
2. Generation of steam for other in-plant uses.
3. Generation of steam for export and/or generation of power for process equipment.
4. Reducing the use of process gas scrubbers as a means of process gas carryover of contaminants.
5. Reducing contaminant carryover into the process gas stream and the deleterious effects on equipment, such as: acids, chlorides, sulphur, and solids.
6. Allowing the operation of the water system to be utilized as a means of cooling and cleaning the process gases.
7. Allowing the flexibility of varying qualities of hydrocarbon fuels used for the varying injection of steam in quantities to reduce deleterious effects from liquid hydrocarbons.
8. Increasing efficiencies of process gas handing compressors by reduction of water vapor throughput.
9. Increasing the availability of compressor use, allowing use of various types of compressors to be used in the process gas handling system.
10. Reducing maintenance and replacement cost of process gas handling equipment from corrosion or erosion.
11. Increasing productivity throughput in process gas availability with the reduction of water vapor requirements.
12. With the recovery of excess heat from exhausted flue gases, increasing the efficiency of flue gas discharge mechanisms, hot fans and/or stacks.
13. Reducing the deleterious effect of contaminants on the longevity of the heat recovery equipment and reformer catalytic reformer tubes and catalyst by the cleaning of the process water used in the scrubber, thereby increasing the safety of equipment operation.
14. Increasing the efficiency of the waste flue gas ejection system by the recovery of excess heat and cooling of the flue gas.
15. Lowering the power requirement of process gas equipment and waste flue gas handling equipment by the generation of steam in exiting flue gas system, and reducing of required water carryover.
16. Enabling the process gas system, reformer system, flue gas system, and reducing gas system to be computer controlled through the separation of the petrochemical area of the process from the metallurgical area of the plant, due to the elimination of water vapor input to the process gas stream which, presently is the basis of control of at least one existing process.
17. Increasing productivity by means of injection of steam, which is readily available and therefore eliminating the startup time to heat water from recycled water streams.
18. Increasing efficiency of present state of the art heat recovery equipment by the reduction of water vapor from the process gas stream.
19. Reducing the quality requirements of steel alloys, therefore lowering the operating and maintenance costs, by the reduction of contaminated materials carryover into the process stream.
One commercially operating direct reduction process requires a ratio control of hydrogen and carbon monoxide at the discharge of the catalytic reformer and prior to injection of gas into the direct reduction furnace of which presently available technology and patents require and stipulate that the method of hydrogen and carbon monoxide ratio control is generated by recycling hot water generated by the process through a process gas scrubber by the saturation of the process gas. Present operation of the prior art process results in water vapor carryover to the downstream gas handling apparatus. This carryover also contains contaminants, such as acids, chlorides, and solids, all having deleterious effects on downstream equipment from wear, corrosion, or erosion, resulting in early failure of equipment, unsafe operation of equipment, and high replacement and maintenance costs.
Present operation of the prior art process also shows process gas handling and flow apparatus, such as compressors, piping, valves, and heat exchangers, to have reduced efficiencies due to the carryover of water. There is also evidence that the water carryover causes flashing in heat exchangers, resulting in unsafe longevity of equipment. There is also evidence that the contaminants and particulates, such as dust, sludge, acidic compounds, chlorides, sulphur compounds, and the like, have deleterious effects on and cause damage to piping systems, compressors, valving, steels and alloys, and catalyst performance, the end result being decreases in performance in machinery, structural integrity of component parts and decreases in efficiency of heat transfer equipment and reforming.
Evidence also appears that slow startup times are required in prior art processes due to the requirement of hot water for the start of operation.
It is believed that the method of hydrogen and carbon monoxide ratio control through the prior art methodology is not exact and is subject to varying degrees with inclement weather, night and day operation, since the method of water temperature control is maintained through a recycled water system.
It is of paramount importance for hydrogen and carbon monoxide ratio to be closely controlled to achieve maximum efficiency in reduction of iron oxides, and therefore full potential of production.
It is also believed that reduction kinetics must be maintained for the raw material iron oxides through the means of exacting hydrogen and carbon monoxide ratio control, otherwise resulting in inefficiencies of operation, furnace clustering of materials, furnace upsets and minimized operations, all of which are evidenced by present operations.
It is also contended that the efficient use of waste energy in the for of exiting flue gas by means of steam generation, recovery of the heat exhausted, allows corrective measures to be taken as claimed, increasing the productivity and efficiency of the overall process.
It is also contended that the present state of the art direct reduction processes commercially utilized are not designed for flexibility in hydrocarbon fuels available and raw material sources available in that they are designed for specific fuel feeds and raw material sources.
The present invention allows for and makes available technology adaptations and implements for the increased efficiency of direct reduction plants and new designs for a direct reduction process for flexibility in both raw materials and fuel sources. While it makes available conversions of existing direct reduction plants for increased capability, production, efficiencies and safer operation with reduced costs.